Rather impressive cold and active late-May pattern developing

After what has already been a rather unusual weather year in California, winter is refusing to give up on the state of California. A highly anomalous deep and cold low pressure area is developing over the far eastern Pacific Ocean and will gradually move south over the next few days, bringing very cool temperatures for this time of year, very windy conditions, and possible convection.

Hi-res visible/infrared hybrid imagery from GOES 11 (GOES Project Science)

The pattern expected this weekend is quite rare for this late in the season. 1000-500 mb thicknesses will be as low as I’ve ever seen them going into June (see below diagram), and there will be sufficient dynamic support to precipitate at least some of the limited moisture that will be available. The ECMWF has done a much better job with the anomalous pattern as of late, and has been indicating for the days the potential for a deep and cold low over CA this weekend into next week. The GFS and NAM are now trending towards the ECMWF, and all of these models now bring very cold temperatures aloft to California over the weekend. In fact, the magnitude of the cold air is pretty consistent from all three models: 850 mb temps could drop as low as -3 C over far NorCal and as low as -2 as far south as Sacramento. The 0C isotherm may progress all the way to southern California on Sunday, which is relatively far south even during the winter months!

Very cold 850 mb temps this weekend (NAM, NCEP)

Strong thermal gradients and differential temperature advection will drive strong winds across much of California over the weekend and into next week. It’s a bit early to pinpoint the location or timing of the strongest winds, but suffice it to say that conditions will be very blustery for late May.


In addition to the wind and cold temperatures, however, is the potential for precipitation and moist convective activity. Temperatures near or below freezing at 850 mb and as high as the mid-60s F at the surface (with strong surface heating resulting from the steep late-May sun angle) will produce very steep lapse rates. The system does appear to have enough overwater trajectory–especially is the ECMWF verifies–to entrain sufficient moisture to sustain surface-based convection. In addition, much of NorCal will be under the left-exit region of the jet on the east side of the trough, with upper-level divergence contributing to substantial UVM. All in all, widespread heavy precipitation is not expected, but at least scattered convection could certainly bring heavy downpours and hail. It’s unclear at this time whether there might be a potential for some severe storms in the Central Valley. This would appear to be most likely in the northern end of the Sacramento Valley, where the complex terrain of the Coastal Mountains can lead to local convergence and higher shear. Due to the extremely steep lapse rates that will be in place, and considering how past late-season cold-core lows have behaved, I think it’s worth keeping an eye out for some stronger convective elements this weekend. Depending on how deep the low can dig, even SoCal could see some showers/isolated thunderstorms by later Sunday. One more note in the precip department: snow levels, believe it or not, may fall below 4000 feet this weekend. That means the Sierra foothills could see some late-May snow showers…

And that’s not the end of the cold and active pattern…not by a long shot, if one is to believe the GFS. An even stronger and deeper system is forecast to slam into CA from the northwest by the middle of next week, this time bringing potentially more rainfall. That would be getting very close to June, so there is some built-in uncertainty with regard to the aclimatological pattern evolution. Right now, though, it looks like we’ll end out the month on a damp and chilly note.

1000-500 mb thickness anomalies ~3 standard deviations below mean (ESRL)

Note: GOES Project Science is a great place to find stunning high-resolution imagery.


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